Electrodes establishing electrical contact with the skin are used for the administration of electrical signals to the body as well as collecting electrical signals generated in the body.
Electrical signals may be administered to the body of a patient through skin electrodes for a variety of reasons, including the treatment of fibrillation by administering an electric shock, treatment of pain and promotion of healing. The electric shock counteracts atrial or ventricular fibrillation of the heart and, if the treatment succeeds, makes the rhythm of the heart revert to the normal mode.
Electric signals generated in the body may be collected by skin electrodes and monitored on a suitable monitoring device. In particular, the electrical signals of the heart may be monitored on a electrocardiogram (in the following abbreviated as ECG) to monitor the operation of the heart.
Skin electrodes have to meet a plurality of requirements to be suitable for supplying or measuring electrical signals, e.g. the skin electrodes must be sufficiently flexible to conform with the patient's body to secure a sufficient contact area, and to display satisfactory adhesion and electrical contact with the patient's body when the electrodes are placed properly. A special requirement consists in the presence of a low impedance to ensure a good transmission of electric energy and a low polarization of the electrode.
If a skin electrode shows a too small ability to transmit energy, the risk exists that the skin of the patient will get burned during defibrillation. Furthermore, during long-term monitoring of ECG, a tendency of the baseline to drift is observed which is believed to be caused by a high D.C. offset. The D.C. offset is a minute current produced by the electrochemical composition of the electrode itself and is influenced by the impedance. The drift decreases the accuracy of the measurements.
A set of skin electrodes may serve the dual purpose of administering an electric shock to a patient liable to fibrillation and monitoring the ECG. It is crucial for the success of the treatment to obtain an ECG as soon as possible after the administration of the electric shock in order to evaluate the effect of the shock treatment. If the electrodes show a high D.C. offset, the ECG trace will be lost for a period lasting from a few seconds to more than a minute after the application of a defibrillation signal because of polarization of the electrodes.
Several prior art documents address the problem of reducing the impedance or increasing the conductivity. In U.S. Pat. Nos. 4,895,169, 4,834,103 and 4,852,585 it is suggested to use an electrode element of tin and stannous chloride. The stannous chloride may be directly applied to the tin surface, e.g. by spraying a thin layer thereon, or it may be located in the conductive medium.
In U.S. Pat. No. 4,352,359, an electrode is disclosed wherein the impedance is lowered by the presence of a synthetic polymer containing at least 5 mole percent of monomer units containing a salt of a carboxylic acid. In EP 0 836 864, it is suggested to use a conductive hydrogel adhesive containing an electrolyte distributed over the foil plate to increase the conductivity, potassium bromide being the preferred electrolyte. In U.S. Pat. No. 4,989,607 it is suggested to use a highly conductive hydrogel comprising a cohesive uniform mixture of poly(vinyl pyrrolidone), a viscosity-enhancing hydrophillic polymer and an electrolyte.
In the prior art, it is often indicated that the corrosion of the metal foil plate is undesired because it reduces the shelf-life of the skin electrode and the compliance with the skin of the patient, cf. for instance U.S. Pat. Nos. 4,834,103, 4,852,585, 4,895,169, 4,674,512 and Danish Patent No 169,235.
The interface between the electrically conductive gel and the metallic layer is of major importance to the conductivity. An effective electrically coupling of the two layers will permit rapid depolarization of the electrode after defibrillation. If the coupling is satisfactory, the electrode will exhibit a relative low and constant D.C. offset.
To be a satisfactory high-performance electrode, the impedance should not vary considerably throughout the used range of frequencies. Especially for ECG applications, low impedances are of importance also at low frequencies. For a typical electrode in the market, e.g. comprising a tin foil coated with a hydrogel containing sodium chloride as an electrolyte, the impedance will rise rapidly when the electrode is subjected to an applied alternating current as the frequency of that current decreases (say 30 kHz to 10 Hz)
The problem of providing a satisfactory electrically coupling between the electrically conductive metallic layer and the electrically conductive gel is attended to herein. Especially, it is the objective of the present invention to reduce the impedance and the D.C. offset to an acceptable low level. Furthermore, it is the objective to reduce the impedance to an acceptable low level at high as well as low frequencies, making the present electrode suitable for a combined use for defibrillation and ECG.